Preheat your oven to 350°F and lightly butter a 9x13 baking dish. Let frozen hashbrowns sit at room temperature for 10 minutes to slightly thaw before mixing, making them easier to combine.

In a large skillet, melt 4 tablespoons butter over medium heat and sauté diced onion and red bell pepper until softened, about 5 minutes. The sautéed vegetables release moisture and create an aromatic base for your cheesy hashbrown casserole.

In a medium bowl, whisk together 6 eggs, 1 cup heavy cream, 1 cup whole milk, garlic powder, smoked paprika, salt, and black pepper until fully combined. This custard mixture ensures creamy texture throughout the baked dish.

In a large mixing bowl, combine thawed hashbrowns, sautéed vegetables, sharp cheddar and Gruyère cheeses, and sour cream. Pour the egg custard mixture over the hashbrown combination and stir gently until every potato piece is coated evenly. This step determines whether your casserole bakes uniformly.

Transfer the mixture to your prepared baking dish and spread it into an even layer. Combine 1/2 cup panko breadcrumbs with remaining fresh chives and sprinkle this mixture across the top for golden crunch.

Bake for 50 minutes until the top is golden brown and a knife inserted into the center comes out clean. The casserole will puff slightly during baking, creating an impressive presentation. Let it rest for 5 minutes before serving to allow the creamy filling to set.